1. Field of the Invention
The present invention relates to a recording material detecting apparatus applied to an image forming apparatus such as a printer or a photocopier, and more specifically, it relates to a reference member for correcting the output from a detecting device.
2. Description of the Related Art
There are many types of sheet materials used as recording materials for image forming apparatuses, for example, so-called plain copying paper (plain paper), transparent film for projectors, white film for obtaining a high-quality image, and so forth. In addition, there are various thicknesses of sheets of plain paper, for example, 60 g/m2 and 200 g/m2 in basis weight.
In order to form an excellent image on any type of sheet material, the image forming condition needs to be changed according to the type of sheet material to be used. For example, in an electrophotographic image forming apparatus, during transferring, an optimum bias needs to be applied according to the resistance value of the sheet material. More specifically, a low bias needs to be applied to plain paper, which has a low resistance value, and a high bias needs to be applied to gloss film, which has a high resistance. In addition, fusing needs to be performed at an optimum temperature according to the heat capacity of the sheet material. More specifically, fusing needs to be performed at a low temperature in the case of plain paper, which has a low heat capacity, and fusing needs to be performed at a high temperature in the case of gloss film, which has a high heat capacity. In order to obtain an excellent image regardless of the type of sheet material, the type of sheet material needs to be detected. In general, an optical sensor is used for such detection.
An optical sensor reads the surface of a sheet material conveyed to the sensor. The accuracy of reading differs from sensor to sensor, and varies due to the accuracy of mounting location, long-term deterioration, and so forth. In order to correct these variations, the sensor reads a reference portion. On the basis of the result, the output from the sensor is corrected. This reading is preferably performed under the same condition as the reading of the sheet material conveyed to the sensor, that is to say, in substantially the same plane as the sheet material (conveying plane). Therefore, the reference portion is preferably placed in substantially the same plane as the sheet material.
However, in this case, the sheet material being conveyed can contaminate and damage the surface of the reference portion.
In order to solve the problem of contamination and damage of a reference portion in an optical sensor, Japanese Patent Laid-Open No. 4-208935 discloses an apparatus in which a reference portion is supported by a movable supporting member and is retracted to a position apart from the conveying surface by a driving unit. That is to say, a supporting member is driven by a driving unit so that a reference portion is retracted to a position apart from the conveying surface, and thereafter a sheet material is conveyed to the conveying surface.
However, in the case of the above-described known apparatus, since a driving unit is used for moving a reference portion, a motor or the like is necessary. Therefore, the cost is increased and the apparatus structure is more complicated.